Recently, polymer based electrically conductive adhesives (ECAs) have been identified as potential alternative electronic interconnects to lead-containing solders in surface mount technology (SMT) applications due to the numerous advantages of ECAs, such as environmental friendliness, mild processing conditions, low stress on the substrates, and good thermomechanical performance. However, some technical barriers concerning the ECAs still restrict their use to only the low-end products (e.g., low power device interconnects). One critical limitation of ECAs that prevents their use for high end products is their unstable contact resistance on non-noble metals, such as Sn, Ni, Sn/Pb. The National Center of Manufacturing Science (NCMS) defined the stability criterion for solder replacement conductive adhesives as a contact resistance shift of less than 20% after aging at 85° C./85% RH conditions for 500 hours, which most conductive adhesives do not satisfy on non-noble metal finishes.
Previous studies indicated that galvanic corrosion was the dominant mechanism for the unstable contact resistance during elevated temperature and humidity aging. The non-noble metal (i.e., a metal with a lower electrochemical potential) acts as an anode and is oxidized to a metal ion (M−ne−=Mn+) by losing electrons. The noble metal acts as a cathode, at which the reaction generally is 2H2O+O2+4e−=4OH−. Then Mn+ combines with OH− to form a metal hydroxide, which is further oxidized to metal oxide. Thus, a layer of metal hydroxide or metal oxide, which is electrically insulating is formed at the interface, therefore causing the dramatic increase in contact resistance of the ECA as it ages.
Based on the mechanism of unstable contact resistance of ECAs, three methods have been applied to prevent the galvanic corrosion of the metal finishes. One is by using oxygen scavengers. Since oxygen accelerates galvanic corrosion, oxygen scavengers could be added into ECAs to slow down the corrosion rate. When ambient oxygen molecules diffuse through the polymer binder, they react with the oxygen scavenger and are consumed. However, when the oxygen scavenger within the ECA is depleted, then oxygen can again diffuse into the interface and accelerate the corrosion process. Therefore, oxygen scavengers can only delay the galvanic corrosion process, but do not solve the corrosion problem.
Another method is to incorporate sacrificial materials with lower electrochemical potential than those of the electrode-metal pads into the ECA. At elevated temperature and high humidity, the sacrificial materials are preferably corroded, and thus, can protect the metal finishes.
Another method to stabilize contact resistance of an ECA is the use of corrosion inhibitors in ECA formulations. In general, organic corrosion inhibitors are chemicals that adsorb on metal surfaces and act as a barrier layer between the metal and the environment by forming a film over the metal surfaces. Thus, the metal finishes can be protected. Some chelating compounds are especially effective in preventing metal corrosion. However, the effectiveness of the corrosion inhibitors is highly dependent on the type of contact surfaces. Although some effective corrosion inhibitors have been developed for Sn/Pb, Cu, and Al surfaces, no corrosion inhibitors for a tin (Sn) surface have been reported.
Recently, Sn has been widely accepted as a surface finish for substrate bond pads in lead-free solder joints due to its low cost and a simple process for the Sn surface finish fabrication. In addition, ECA joints with Sn are less susceptible to Sn whiskering, compared to Sn alloy solder joints. However, tests showed that conventional ECAs/Sn joints exhibited considerably unstable contact resistance at early stages under aging environment. Therefore, the development of effective corrosion inhibitors for a Sn surface is of great significance and interest within the art.